This invention relates to latex-modified concrete structures. More specifically, this invention relates to latex-modified, pretensioned and prestressed concrete structures having enhanced structural load bearing capacity and adapted for applications in exterior construction work such as slabs, beams, arch ribs, girders, restraining walls, pavings, and columns, where excessive vibration and/or heavy load may be encountered. A preferred use of the structures of this invention is as railroad crossties. Such crossties advantageously replace wooden ties conventionally used to support rails on a railway roadbed composed of particulate ballast.
The present invention will more particularly be described hereinafter in terms of novel railroad crossties without intending to limit its scope.
Although wood ties have been and continue to be generally used in the railroad track construction and maintenance, other materials have been sought and suggested for use particularly where the inherent characteristics of wood make wood ties unsatisfactory or where the scarity or cost of suitable timber for wood ties makes substitute materials attractive. To this end, ties fabricated of prestressed concrete have been introduced.
The term "pretensioned, prestressed concrete" as used herein and generally in the art means reinforced concrete structures wherein the concrete is under compression and the reinforcing element such as steel is under tension. The reinforcing element is stressed or put under tension before the concrete is poured.
Although the prestressed concrete ties have solved many problems associated with wood ties, some new problems unique to the concrete ties have been encountered. More specifically, the concrete ties are extremely heavy or awkward to handle and are known to cause loss of electrical currents transmitted through the rails to control modern signal systems because of relatively low resistivity of concrete as compared to wood. One of the most serious problems associated with prestressed concrete railroad ties is cracking when such ties are subjected to flexural loading, i.e., when sufficient weight is applied to the ties to impart a flexural deformation thereof. In recent years, the cracking problem has further been aggravated with increasing speeds and loads of trains. Cracking of prestressed concrete is due primarily to the loss of prestressing force resulting from inadequate bond transfer from the prestressing reinforcing element such as steel to the surrounding concrete matrix. Because of the relatively high bond transfer length of prior known prestressed concrete crossties, a large number of relatively small prestressing steel wires or rods is required to achieve adequate bond transfer between the steel reinforcement and the concrete. The use of a large number of reinforcing wires or rods is commercially undesirable. Furthermore, it has been necessary to over-design concrete crossties in order to have good bond transfer between the steel wires or rods and the concrete, resulting in a crosstie design of excessive weight.
The term "bond transfer length" as used herein and generally in the art means the distance over which the stress in a pretensioned strand is transferred by bond to the surrounding concrete. When a good bond transfer exists between the strand and the concrete, the bond transfer length is relatively short, and vice versa.
Prestressed concrete structures have been known in the art for many years, e.g., as disclosed in U.S. Pat. Nos. 1,684,663; 2,080,074; 2,535,100 and 3,086,273. Railroad ties produced from prestressed concrete are also well known in the art, e.g., as disclosed in U.S. Pat. Nos. 2,538,443; 2,538,437; 2,695,754 and 3,687,346; as well as Polish Pat. No. 44,184 and Pakistanian Pat. No. 111,699.
Modification of portland cement compositions with vinylidene chloride polymer latexes of the type specified herein is disclosed in British Pat. No. 967,587. Additional prior art directed to the modification of portland cement compositions with vinylidene chloride polymer latexes include U.S. Pat. Nos. 2,819,239; 3,239,479; 3,411,943; 3,773,700; 3,879,909 and Ser. No. 815,967.
Heretofore, however, it has not been known to utilize portland cement compositions containing the herein specified types and amounts of vinylidene chloride latexes for the preparation of prestressed concrete structures having improved bond transfer between the concrete and reinforcing steel wires.
Accordingly, it is an object of this invention to provide prestressed concrete structures, e.g., railroad crossties, having improved bond transfer between prestressing reinforcing elements, such as, for example, steel wires, and concrete which structure is lighter in weight, has enhanced structural load bearing capacity, and can be fabricated with fewer prestressing elements having relatively large crosssectional area. Other objects and advantages of the invention will become apparent in the following description.